In cone-beam computed tomography (CT) systems, an x-ray source and an x-ray detector are generally mounted on opposing ends of a substantially C-shaped gantry such that x-rays emitted by the source in a cone-shaped beam are incident on and detectable by the x-ray detector. The source and the detector are positioned such that when an object (e.g., a human extremity) is interposed there between and is irradiated with x-rays, the detector produces data representative of characteristics of the interposed object. The data produced is typically displayed on a monitor or electronically stored.
The C-arm gantry defines an axis of rotation about which the source and detector are rotatable. By positioning this axis of rotation at or near an object, and by rotating the source and detector around the object in an orbital motion, images of the object taken at a plurality of different orientations can be obtained. These images can be combined to generate a comprehensive three-dimensional image of the object, for example using methods of image reconstruction.
Such CT system configurations typically have a small field of view and thus can only image a small portion of an object during a single scan. When imaging an off-center portion of an object, say a liver of a patient, the table upon which the patient rests during the scan is typically positioned such that the anatomy of interest coincides with the field of view. However, it is possible that the detector and/or the source may collide with the patient because the patient is now positioned closer to the trajectories of the detector and the source. Currently, imaging system operators use a trial-and-error approach wherein the patient is repositioned so that no such collisions occur. In some instances, repositioning the patient may lead to the anatomy of interest lying outside of the imaging field of view. If the patient is imaged in these instances, then the patient needlessly receives radiation dose because the anatomy of interest is not actually imaged. Typically, finding the optimal position of the subject where no collisions occur and the anatomy of interest is successfully imaged can take up to fifteen minutes of time.